1. Field of the Invention
The present invention relates generally to a liquid crystal display device, and is directed more particularly to a liquid crystal display device in which the life span of a liquid crystal display used therein can be greatly prolonged.
2. Description of the Prior Art
Since a liquid crystal display device consumes little electric power and can be driven by a low drive voltage, it is used as various display devices such as numerical display, letter display, bar graphical display, television video display, and so on.
It is known that if the liquid crystal is driven by a DC voltage, its life span is deteriorated or greatly shortened, so that in general the liquid crystal is driven by an AC voltage. In the latter case, if any DC voltage component remains across the liquid crystal, its life span becomes gradually shorter. Since the remaining or residual DC voltage component badly effects the life time span of the liquid crystal even if its value is in the order of mV (milli-volt), it is desired to suppress the residual DC voltage across the liquid crystal lower than 50 mV. In fact, even if the remaining DC component across the liquid crystal is relatively low, this remaining DC component is accumulated and then badly affects the liquid crystal.
In a prior art liquid crystal drive method, a liquid crystal cell is directly supplied at one of its opposing electrodes with a drive waveform from an AC drive source terminal and at its other electrode with the same through an exclusive OR gate circuit. With such a prior art method, the duty cycle of the drive waveform is deformed and hence a remaining DC component for the applied voltage is applied across the liquid crystal cell with the result that its life time span is deteriorated or becomes short. The above defect especially appears as the drive frequency becomes high.
In another prior art liquid crystal drive method, by means of a resistor division, the mid-point potential of the drive waveform is applied across a liquid crystal. In this case there may occur a DC offset due to the resistive error of the resistors and hence a DC component will remain across the liquid crystal.
It may be considered as another drive method for a liquid crystal that a non-polar capacity is provided so as to prevent a DC component from remaining. In this case, however, it is necessary to use a capacity and to provide a non-polar capacity for each of a plurality of liquid crystal cells (segments) which are arranged to be, for example, a figure "8" or a predetermined pattern in parallel relation so as to achieve various displays such as a numerical display, bar graphic display and so on. Therefore the construction becomes large, complicated and expensive. In this case, since the remaining DC component is consumed by a resistor, the electric power consumed by the whole device becomes great.